General
Coherent light radiation is converted, amplified and generated to obtain light signals for use in telecommunications, spectroscopy, medicine and so on and to obtain wavelength-division-multiplexing (WDM) signals for use in telecommunications. Unfortunately, the lasers used as sources of coherent light usually have an almost fixed wavelength that can be changed in a relatively narrow wavelength range. An optical parametric converter comprising an optical non-linear crystal (LiNbO3, KTP etc) can convert light from a laser having a fixed wavelength into coherent light having one or more different wavelengths. An optical parametric converter uses one light signal as a feeder, or pump, to convert, or amplify, a second light signal. When a pump light signal with frequency xcfx89p and a second light signal with frequency xcfx89s are launched in specific directions into a non-linear crystal, those signals interact to generate a signal with frequency xcfx89s+xcfx89p or |xcfx89sxe2x88x92xcfx89p|. Known optical parametric converters require a powerful coherent light radiation pump, such as, for example, a powerful gas or solid state laser. These lasers are large, heavy, and costly and have a short life.
In the field of acoustooptics, there are known optical devices in which an acoustic wave interacts with a light wave. In most known acoustooptic devices, a light wave""s wavelength is preserved. Usually, the spatial distribution of a light wave changes as a result of its interaction with an acoustic wave. However, there are known acoustooptic devices in which the wavelength of a light wave is expressly changed. One such device is based on an acoustooptic Bragg cell. In this device, the frequency of a light wave transmitted through the device is equal to xcfx89+xcexa9, where xcfx89 is the frequency of the light wave and xcexa9 is the frequency of the acoustic wave. That is, in such a device, the interaction of a light wave with frequency xcfx89 and an acoustic wave with frequency xcexa9 produces a light wave with frequency xcfx89+xcexa9.
Another device using converters to directly convert light waves in a fiber has been described. A converter with properties analogous to this device has been disclosed by Risk, et. al. in U.S. Pat. No. 4,872,738 issued Oct. 10, 1989. Like the acoustooptic Bragg cell device, the frequency of the output wave in the Risk device is xcexa9+xcfx89.
In Russian Patent #2085984 entitled Parametric Amplifier and Converter for a Wavelength of Electromagnetic Radiation, V. P. Torchigan disclosed an acoustooptic device that increases the frequency of an output light wave.
The theoretical considerations of the interaction of an ultrasonic wave and a light wave in a modified Bragg cell in the form of a truncated pyramid have been discussed in a paper by Torchigin. Enlarged shift of light frequency in a modified Bragg cell. Torchigin, V. P. and Torchigin, A. V. Pure Appl. Opt. 7 (1998) 763-782.).
An acousto-optic cell (AO cell) with constant cross-section is a well known acousto-optic device that is used widely for switching, modulation, filtering light radiation, light beam deflection and scanning.
In an acousto-optic device, under the proper conditions, the spatial distribution of a light wave may be changed as a result of the interaction of a light wave with an acoustic wave. Such a change in the spatial distribution of a light wave may occur, for example, when a light wave, which usually propagates in free space, is launched in an AO cell, and the light wave interacts with the acoustic wave over a distance of about 1 cm with an interaction time of less than 0.1 ns.
Compared to free space optical configurations that require relatively large amounts of space and materials, integrated optics decrease the size and increase the reliability of acousto-optic devices. Such is the case, for example, in devices in which light waves are propagated in a planar waveguide of a few micrometers thickness.
In the present invention, the same structure acts as a light guide for the light wave and a sound guide for the acoustic wave.
Theoretical
Theoretical considerations are helpful in understanding and explaining the beam deflection used in the novel devices discovered and disclosed by the inventor.
Consider a waveguide in which both an acoustic wave and a light wave are launched. When the wavelength of the light wave is less than the wavelength of the acoustic wave by a factor of about 100, there is, usually, no noticeable interaction between the waves in the waveguide. However, if the light wave has a xe2x80x9cwhispering galleryxe2x80x9d type waveguide mode, its wavelength along the axis of the waveguide can increase significantly enough that the interaction between the waves can be very strong. In addition, varying the cross-section of the waveguide results in other advantages.
Since an AO cell with variable cross-section is considered to be a waveguide, the means for entering light radiation in an AO cell are similar to the means used for entering light radiation into waveguides. There are known means for entering light radiation through an end face of a waveguide and various coupling devices for entering light radiation through a side surface of a waveguide. Both types of means are used in the present invention. The type of coupling used depends on the specific application.
In general, in this disclosure, an AO cell is a sound guide. Typically, the sound guides used in this invention are wave guides suitable for functioning as both sound guides and light guides.
In a conical waveguide (usually called a focon or focusing cone) fabricated from a homogeneous linear isotropic optical medium, even a very small change in the direction of an input beam incident on the wide face, or base, of the focon, results in a significant change in the direction of an output beam reflected from the wide face, or base, of that focon.
Since a modified AO cell with variable cross-section along its axis is a waveguide with variable cross-section, the known peculiarities of the propagation of light waves in a waveguide with variable cross-section are applicable.
Geometrical Optics Approximation
Consider an input beam of light that enters a focon at a point on the base near the side surface of the focon. If the direction of the input light beam lacks a radial component (that is, the projection of the input light beam on a radius that extends from the center of the base of the focon through the entry point of the light beam equals zero) and the light beam first strikes the side of the cone at a point just below the base, the trajectory of the beam in the focon looks like a conical spiral with decreasing intervals between its coils as the beam travels from the base towards the narrow end of the focon.
The beam spirals inside the cone to a point R, where R is the point of maximal penetration of the beam into the cone. The distance from R to the vertex O of the cone is determined by the equation OR=xcfx81[1xe2x88x92Sin xcex1], where OR is the length of the straight line drawn from the vertex of the cone to the point of maximal penetration, where A is the point at which the input beam first strikes the side surface of the cone, xcfx81 is the length of the straight line drawn from the vertex of the cone to the point A, and xcex1 is the angle that the input light beam makes with the straight line OA.
Once the beam reaches maximal penetration, the beam reverses direction relative to the axis of the focon and travels towards the base. That is, the point of maximal penetration is also a return point. The angle that an exit beam makes with the straight line OB drawn from the vertex to the point B, where B is the point at which the beam travelling within the focon last strikes the side surface of the cone, is the same angle xcex1 that the incident input light beam made with the straight line OA. The location of exit point B of the output beam varies dependent on the angle xcex1. That is, there exists an angle "psgr" such that xcex94"psgr"=2xcex94xcex1/xcex3 (where xcex3 is the taper of the focon) whereby as xcex1 changes "psgr" changes and the exit point B moves along a circle drawn on the side surface of the focon near the base of the focon. That is, the direction of the azimuth component of the point B varies in the range 0 . . . 360xc2x0, as xcex1 changes. Thus a small change in the angle xcex1 results in a significant change in the location of the exit point.
Now let a travelling azimuth independent acoustic volume wave with longitudinal oscillations propagate along the axis of the focon towards its tapered end. As is known, due to photo-elastic effects, an acoustic wave changes the refraction index n of the medium in which it propagates. As a consequence of this phenomenon, in a focon, an acoustic wave forms a periodic structure that moves at the velocity of the acoustic wave. This structure acts like a distributed Bragg reflector (DBR). As is known from acousto-optics, a travelling plane light wave reflects from a plane acoustic wave if the following so called Bragg condition (kK)=K2/2 is satisfied, where k is the wave vector of the light wave and K is the wave vector of the acoustic wave. (The point at which the condition (kK)=K2/2 is satisfied is a return point.) The same effect occurs in a waveguide with a periodic structure and is used widely in the design of waveguide filters. The Bragg condition can be rewritten in the following form 2k Cos xcex8=K where xcex8 is the angle between the wave vectors k and K. In a focon, the angle xcex8 tends to 90xc2x0 as the light beam travels to the return point.
Usually the wavelength of the acoustic wave xcex9 is much greater than the wavelength of the light wave xcex. Since k=2xcfx80/xcex and K=2xcfx80/xcex9, K less than  less than k, the Bragg condition is satisfied when xcex8 is close to 90xc2x0. In a focon, the angle xcex8 corresponds to the angle between the axis of the focon (the direction of the wave vector of the acoustic wave) and the direction of the light beam (the direction of the wave vector of the light wave). It follows that, in a focon, there always exists a point where the angle xcex8 is close to 90xc2x0 and the Bragg condition is satisfied. Thus, in a focon, the position of the return point of a light beam can be changed by the presence of an acoustic wave. The distance r between the new return point (that is the return point when an acoustic wave is present) and the vertex of the cone is determined by the equation r=R/Sin xcex8, where xcex8 is determined from the equation 2k Cos xcex8=K. Thus, the presence of an acoustic wave moves the return point closer to the base. As a result, the trajectory of a light beam and its exit point can be changed by the presence of an acoustic wave. In this manner, the acoustic wave can control the light beam.
The geometrical optics approximation takes known acousto-optics phenomena into account and assists in understanding the main idea and principles upon which the apparatuses disclosed in this invention are based. The general case where the apparatus comprises a body of rotation with variable diameter along axis can be described using a wave optics approach.
Wave Optics Approach
Using a waveguide approach, an AO cell can be thought of as a waveguide with a slowly-variable cross-section and a xe2x80x9cwhispering galleryxe2x80x9d mode travelling light wave can be thought of as a travelling light wave that rotates around the axis of the waveguide and shifts slowly in an axial direction. When total reflection from the side surface of a body of rotation is present, the waveguide confines a light wave.
It is known that the waveguide mode of an electromagnetic wave is preserved when the wave propagates along a waveguide with slowly-variable cross-section. This means that the number of variations of the electromagnetic field in cross-section is constant and, therefore, the period of the electromagnetic field in the cross-section xcexxe2x8axa5 is proportional to its diameter D, i.e. xcexxe2x8axa5.xcx9cD. Since projection kxe2x8axa5 of wave vector k on the plane of a cross-section varies in inverse proportion to xcexxe2x8axa5, kxe2x8axa5xcx9c1/D. The dependence of the propagation constant k=(z) of a light wave along the axis of the waveguide on its position z along that axis can be obtained from the following relations
k2xe2x8axa5(z)xe2x88x92k2=(z)=k2, xe2x80x83xe2x80x83(1xe2x88x92i)
k2xe2x8axa5(z)=k2xcfx86(z)+k2r(z)xe2x80x83xe2x80x83(2xe2x88x92i)
kxcfx86(z)/kr(z)=kxcfx86(0)/kr(0), xe2x80x83xe2x80x83(3xe2x88x92i)
kxe2x8axa5(z)D(z)=kxe2x8axa5(0)D(0), xe2x80x83xe2x80x83(4xe2x88x92i)
k=(0)=k Cos xcex8xe2x80x83xe2x80x83(5xe2x88x92i)
where kxe2x8axa5(z) is the component of the wave vector that is perpendicular to the axis, kxcfx86(z) and kr(z) are the azimuth and radial components of the wavevector, respectively, D(z) is the diameter of the waveguide at point z, xcex8 is the angle of incidence of the light beam on the base of the waveguide at z=0. If the dependence of D(z) on z, the position along the axis of the focon, is known, one can determine the dependence propagation constant of a light wave in the waveguide k=(z). Then, the location of the return point on the z-axis can be determined from the condition k=(z)=K/2.
The limitation on the radial component of an incident input beam, introduced above in a geometric optics approximation for the sake of simplicity and understanding only, is not needed when a waveguide approach is used. In fact, the existence of a radial component kr improves the interaction between acoustic and light waves because the greater is kr the wider is the ring in which light radiation is present. This results in an increase of the overlapping of the acoustic and light waves. However, the increase of kr is limited by the condition of total reflection from the side surface of the waveguide. That is, the condition kr less than k(n2xe2x88x921)1/2 must remain valid, where n is the index of refraction of the optical medium from which the waveguide is fabricated.
Generalization
The foregoing waveguide theory can be applied to a focon and other bodies of rotation, as well as to structures having a regular polygonal cross section and structures having cross sections that satisfy the condition of having total reflection of light ray trajectories from their side surfaces.
It is important to determine the relation between the taper of a focon and the intensity of an acoustic wave. It is known from DBR theory that the greater the index of modulation m of the refractive index n, the greater is the index of reflection xcex93 of the DBR. Also, the less detuning xcex4=k=xe2x88x92K/2 the more xcex93. The ability of a DBR to reflect disappears when xcex4/k= greater than m.
On the other hand, in a focon, k= is not constant and depends on z. The greater the taper of the focon, the greater the change in k=(z) along the z axis (that is, the longitudinal axis of the focon or body of rotation) and lesser the length of the DBR, i.e. the segment of the waveguide where xcex4/k= less than m. Thus, in order to preserve the ability of the DBR to reflect, an increase in the taper of the focon must be accompanied by an increase in the index of modulation m. The known DBR theory is valid only for small m and xcex4. Because a focon is considered to be a waveguide whose cross-section changes along its axis, the index of modulation M of the propagation constant k= must be used instead of the index of modulation m. As is shown in the paper Torchigin V. P., Torchigin A. V. Enlarged shift of light frequency in a modified Bragg cell. Pure and Applied Optics, vol.7, 763-782 (1988) M greater than  greater than m in the vicinity of the return point. In this case, the DBR theory for small m and xcex4 is not valid and a direct solution for the propagation of light in a waveguide with a variable propagation constant along the axis of the waveguide is required. An approximate value of the index of modulation m required for operating can be obtained from the relation m="sgr" where "sgr"=(dD/dz)/(2xcfx80D/xcex9), where xcex9 is the wavelength of the acoustic wave.
Those interested in a more detailed description of the theoretical considerations presented in the foregoing paragraphs are referred to the paper by Torchigin V. P., Torchigin A. V. Enlarged shift of light frequency in a modified Bragg cell. Pure and Applied Optics, vol.7, 763-782 (1988).
General
The Torchigin paper discusses frequency conversion, amplification and generation of light radiation by means of an acoustic wave. The present invention discloses additional findings in those areas and introduces applications in the areas of deflection, modulation switching and light radiation filtering.
Note that in this disclosure a focon is often used for simplicity of understanding. However, the embodiments disclosed will function with a body of rotation provided that the limiting criteria are met. And the inventor intends that the disclosure be read with this generalization to a body of rotation in mind. Also note that the terms axis of a focon and z-axis are used interchangeably to denote the axis drawn from the base of a cone to its vertex, or imaginary vertex in the case of a truncated cone.
Furthermore, although the embodiments disclosed can function as stand alone apparatuses it is to be noted that they can also function as a section of another apparatus. For example, a tapered cone segment can be incorporated into a fiber or glass cable.
The invention is suitable for use in various applications and the particular preferred embodiment depends on the particular application.
An objective of this invention is to provide methods and apparatuses for deflecting, modulating, switching and filtering light radiation by means of an acoustic wave. The invention facilitates and controls the interaction of an acoustic wave propagated along the axis of a wave guide and a xe2x80x9cwhispering galleryxe2x80x9d mode light wave introduced into that same wave guide to achieve the desired results, such as, for example, the deflecting, modulating, switching and filtering of light radiation.
The invention discloses how selection of acoustic wave parameters, such as amplitude and frequency, and the profile of the wave guide can be used to manipulate the propagation of light waves to achieve the desired results.
In general, the apparatus comprises a body, an optional acoustic transducer or other means of introducing an acoustic wave into the body, at least one light input means and at least one light output means.
Optionally, the body is a body of rotation. The body is made of a suitable glass material. In this application, the terms xe2x80x9cglassxe2x80x9d and xe2x80x9csuitable glass materialxe2x80x9d are used generically to indicate any medium, suitable for the transmission of light, in which the rate of attenuation of light radiation is sufficiently low to permit conversion or amplification or other functions of the apparatuses disclosed herein. For example, in a disclosed apparatus that requires the interaction of an acoustic wave and a light wave, the suitable glass material is one that permits said interaction. The glass may be homogeneous or inhomogeneous. At least part of the body is tapered. A tapered part of a body extends from a broad base to a narrow end and comprises an axis extending from the broad base to the narrow end. An example of such a tapered part of a body is a truncated cone, also known as a focon. Preferably the tapered part tapers gradually from a broad base to a narrow end. Also, preferably, the tapered part is symmetrical with respect to a central axis that extends from the centre of the broad base to the centre of the narrow end. Furthermore, it is preferable that, in any cross section cut perpendicular to the axis of the tapered part, the index of refraction decreases as the distance from the axis increases. It is also preferable that any cross section cut perpendicular to the axis of the tapered part of the body is either a regular polygon or a circle. Suitable shapes for the body include cylinders, cones and pyramids. A cylinder may have an elliptical base or any polygon as a base. Thus, recalling that a circle is an ellipse in which the axes of the ellipse are equal, an elliptical cylinder includes a tube with a circular base. A common example of the use of a cylinder with a polygonal base (i.e. a rectangular cylinder) is a milk carton. Suitable symmetrical shapes for the body include, but are not limited to, cylinders and cones with circular or elliptical bases and pyramids with square, rectangular, and regular hexagonal bases. The narrow or tapered end of a cone or pyramid used in this invention may be truncated. Optionally, the narrow or tapered end of a body is truncated. Also, optionally, the tapered or truncated end comprises a matching load. Furthermore, the body may optionally comprise a hollow or solid core and the refractive index of the core may be different from the refractive index of the remainder of the body. Also, optionally, the body may comprise a coating that has a different refractive index from the remainder of the body. Furthermore, said coating may provide a reflective surface for the remainder of the body.
Preferably, the acoustic wave introduced into the body is either a Lamb wave or a surface acoustic wave or a travelling acoustic wave.
Preferably, the electromagnetic field of a light wave input into the body is in the form of a waveguide mode of the xe2x80x9cwhispering galleryxe2x80x9d type. One input means suitable for such light wave input is an optical coupling prism.
In a given embodiment, a light input means may be closer to or farther from the base of a tapered part of the body than an output means, such placement dependent on the purpose of the apparatus of the particular embodiment.
A feature of a tapered part of the body of the invention is that the taper is sufficiently gradual to permit the function of the particular embodiment disclosed. In the truncated cone, the taper can be calculated from equation (3), namely, "sgr"=(dD/dz)(xcex9/2xcfx80)Dxe2x88x921. One knowledgeable in the art will recognise that corresponding equations for the taper can be used for the particular shape of the tapered part being used in a given embodiment. It is important to note that as long as the tapering equation corresponding to the particular embodiment is satisfied, the principles disclosed in this application are valid. Thus, although the embodiments are typically described using a truncated cone as the body or part of the body, one skilled in the art will recognise that the body can take on any shape, and need not be symmetrical. Accordingly, for example, the principles disclosed herein can be employed in constructing a flexible cable that has a cross section that varies irregularly along its length and that curves as needed by the particular connections required.
For simplicity, the term xe2x80x9cbodyxe2x80x9d is used in the descriptions of the embodiments of this invention, but one skilled in the art will recognise that the embodiments disclosed may be a section or a part of another apparatus. For example, a cable may comprise one or more truncated conical parts or sections interspersed along the length of the cable.
One should also note this invention discloses various means to confine light and acoustic waves. Advantages of the confinement of light or acoustic waves include permitting one to take advantage of various properties of those waves, limiting energy losses from those waves, and, when both acoustic and light waves are present simultaneously, enhancing the interaction of those waves.
Technical Principles
The discussion that follows will be useful for an understanding of the prior art and the theory of light conversion in parametric systems with very low pump frequency as applied to the inventions herein disclosed.
For simplicity, the principles in the following paragraphs are discussed in relation to simple structures, such as, for example, a focon. However, these principles are applicable to more complex structures provided that the corresponding conditions are satisfied, as is the case, for example, when an equation corresponding to a complex is substituted for the equation disclosed for a simple structure.
The invention disclosed herein takes advantage of the development of glass with extremely small losses (for example, less than 0.2 Db/Km) for the conversion and amplification of light. Such glass is used in the fabrication of optical fibers. In this application, the term xe2x80x9cglassxe2x80x9d is used generically to indicate any medium, suitable for the transmission of light, in which the rate of attenuation of light radiation is sufficiently low to permit said conversion and/or amplification. As the dissipative losses increase the efficiency of conversion decreases. As the dissipative losses of the glass in an acousto-optic device decrease, the time of conversion in that device increases and the frequency of the pump in that device decreases. Although glasses are available in which the attenuation of light radiation is less than 0.2 Db/Km, depending on the application, greater dissipative losses can be acceptable.
In Russian Patent #2085984 (2085984RU), Torchigin disclosed means for the conversion and amplification of light radiation. In Patent 2085984RU, conversion is performed in a truncated glass cone (or focon), wherein an acoustic transducer attached to the wide base of the focon excites a traveling acoustic wave that propagates along the focon and is absorbed by a matching load attached to the tapered end of the focon. A matching load absorbs an acoustic wave and decreases, and preferably eliminates, reflection of the acoustic wave. Light radiation enters the focon through an input coupling prism attached to the side surface of the focon and light radiation is extracted from the focon through an output coupling prism attached to the side surface of the focon.
The acoustic transducer generates a traveling acoustic wave with a frequency of about 100 MHz. When propagated in a focon, a traveling acoustic wave changes the pressure of the medium through which the acoustic wave propagates. As the refractive index of the medium depends on the pressure in the medium, a change in pressure results in a change in the refractive index. In effect, as the acoustic wave travels in a focon, there is a wave of change of the refractive index of the medium. This wave of change travels at the same phase velocity as the acoustic wave, about 6000 m/s. The distance between adjacent (consecutive) peaks along the longitudinal axis of the focon is equal to xcex9, where xcex9 is the wavelength of the acoustic wave. Light radiation that enters a focon through an input coupling prism excites a light wave that has a xe2x80x9cwhispering galleryxe2x80x9d mode (WGM) electromagnetic field. As a result of the parametric interaction between the acoustic and light waves, the frequency and energy of the light wave increase. That is, under suitable conditions, after a light wave interacts with an acoustic wave, the energy of the resultant light wave equals the summation of the energy of the initial light wave plus the energy extracted from the acoustic wave. As the resultant light wave propagates under an output coupling prism energy is extracted from the resultant light wave and is used as an output signal.
Note that in the discussion that follows different descriptions (time of interaction, time of conversion, etc.) of xcfx84 are used for clarity of explanation and are interchangeable.
A radical decrease in pump frequency does not contradict the physics of parametric interaction. It is known from the physics of parametric amplification that the energy of an amplified signal in a lossless parametric system increases by a factor of about (1+2xcfx80m) over one period of modulation or one period of the pump, where m is the index of modulation of a parameter that characterizes the energy stored in the parametric amplifier. Then, the energy of the light signals increases by a factor of (1+2xcfx80m)1/2 xcfx80m≅e over
N=1/2xcfx80mxe2x80x83xe2x80x83(1)
periods of modulation. We can assume that the strength of parametric interaction S is characterized by the time xcfx84 that is required to increase the energy of a light signal e times. As xcfx84 decreases S increases. In the case under consideration, xcfx84=1/(2xcfx80mF)=1/(mxcfx89p). Thus, as either m or xcfx89p increases, the strength of parametric interaction increases. It is not surprising that an increase in either m or xcfx89p is accompanied by additional difficulties.
In this context, the question arises as to what degree decreasing the strength S is allowable? Increasing the time of interaction xcfx84 entails increasing light losses. Therefore, the strength S can be decreased until the efficiency of parametric amplification is acceptable.
A clearer understanding is obtained when concrete values of the index of modulation m and the pump frequency xcfx89p are used. For example, if m=10xe2x88x924, the energy of light radiation increases by a factor of e, independently of the value of the pump period T, in 104/2 xcfx80≅1500 periods of the pump. If a prior art optical parametric amplifier has an optical pump with wavelength xcex0=0.6 xcexc, then the period of the pump is T=2*10xe2x88x9215 s and the time of conversion is xcfx84=1500 T=3*10xe2x88x9212 s. Since the losses in a nonlinear crystal are about 0.2 Db/cm, or about 0.6*1010 Db/s, the losses in time xcfx84 equal 0.018 Db. Therefore, the time of amplification in the nonlinear crystal can be increased by a factor of about 100, at least. On the other hand, if the frequency of a pump equals 100 MHz, then 1500 periods occur in 15 xcexcs. Because the group velocity of a light signal in the medium is about 2*108 m/s, the light signal transverses 3 Km in 15 xcexcs. Until recently, there was no optical medium through which light could transverse such a large distance without evident losses. Therefore, it was impossible to use a pump having a significantly decreased frequency F. The situation changed radically when the glasses for the fibers used in telecommunication were developed. Light losses from these fibers are minimal, about 0.2 Db/Km.
Although, glass fiber is not a nonlinear medium, it is known from the principles of acoustooptics that its refractive index m can be modulated. This modulation can be explained by the photo-elastic effects that are present in frequency ranges up to a few gigaherts.
Thus, simple calculations demonstrate that a low frequency pump can be used for conversion and amplification of high frequency radiation provided that total losses are acceptable for a given situation. However, these considerations are not useful in determining what optical device can be used for this purpose.
In a focon, regions in which the refraction index has been increased by one or another means function like an open dielectric resonator. It turns out that these regions can store light radiation launched into them. The best known specimen of this type of resonator is a small glass ball with a diameter of about one millimetre or less. Similar resonators, such as xe2x80x9cwhispering galleryxe2x80x9d mode resonators, have a very large quality factor of about Q=109. Such resonators are used, for example, in semiconductor lasers to provide high stability of the output light frequency.
A resonator mode of the xe2x80x9cWhispering Galleryxe2x80x9d type can be thought of as a light wave that travels along the equator of a ball. The total length of a light wave travelling along the equator of a glass ball is equal to an integer number of the wavelength of that light wave in glass. And, in general, the total length of a light wave travelling around the equator of a sphere made of a given medium is an integer number of the wavelengths of that light wave in that medium.
Generally, a resonator mode of the xe2x80x9cWhispering Galleryxe2x80x9d type can be obtained on any cylindrical surface or barrier with a smooth curvature. Accordingly, the invention herein disclosed is applicable to waveguides with smooth curvilinear surfaces.
For the sake of simplicity, consider one of the simplest resonator modes of the xe2x80x9cWhispering Galleryxe2x80x9d type, namely, a resonator mode in which a light field is concentrated in a narrow band (strip) near the equator of a ball and does not penetrate deeply inside the ball. Indeed, the tendency of a light wave to propagate straight forward causes the light wave to strike against the outer surface of the ball. As a result, straight-forward propagation of the light wave is impossible due to numerous sequential reflections of the light wave from the spherical glass-air boundary.
The confinement of a light wave to a region near the equator and its absence in regions near the poles is explained by the peculiarities of a light wave""s reflection from a spherical surface. Any light wave deflected from the equatorial plane reflects so that the deflection decreases or reverses direction. That is, any light wave deflected from the equatorial plane reflects off the surface of the sphere back towards the equatorial plane. The fact that the diameter of a cross-sectional plane that is parallel to the equatorial plane decreases as the distance from the equator increases contributes to confining a light wave in the region near the equator.
Another example of the use of a resonator mode of the xe2x80x9cWhispering Galleryxe2x80x9d type is an open dielectric resonator in the form of a disk. Similar resonators are widely used for electromagnetic waves having a wavelength in the millimetre range. As in the ball, due to the phenomenon known as total internal reflection, the propagation of a light wave along a radius of a disk is confined by reflections from the cylindrical surface of the disk. Furthermore, propagation of a light wave along an axis of a disk is confined by the two circular surfaces of the disk. The wave reflects from the circular surfaces of the disk due to the same phenomenon of total internal reflection. Thus, the wave is completely confined by the surfaces of the disk.
The same result is obtained when one thinks of a disk as a planar dielectric waveguide (such as, for example, an infinite plane film or plate) confined by a cylindrical surface, where the axis of the cylinder is perpendicular to the plane of the waveguide. The propagation of a light wave in a disk is similar to the propagation of a light wave in a planar waveguide, except that, in the disk, the cylindrical surface of the disc, similar to the spherical surface of the ball, confines propagation of the resonator mode of the xe2x80x9cwhispering galleryxe2x80x9d type in the radial direction.
The greater the index of modulation m=xcex94n/n of the refractive index of an acoustic wave and the lesser the taper of a focon, the more favourable are the conditions for confining light radiation by a dielectric resonator. In the limiting case, when the frequency of an acoustic wave F is small enough the condition takes the following simple form
m greater than "sgr", xe2x80x83xe2x80x83(2)
where "sgr" characterises the taper of the focon and is determined as follows
"sgr"=(dD/dz)(xcex9/2xcfx80)Dxe2x88x921xe2x80x83xe2x80x83(3)
where D is the diameter of the cross-section of the focon, z is distance along the z-axis of the focon, xcex9=va/F, va and F are the velocity and frequency of the acoustic wave, respectively.
As follows from (3), "sgr" is equal to the relative change of the diameter of the focon xcex94D/D at distance xcex9/2xcfx80. Thus, in accordance with (2), moving open dielectric resonators can appear in a focon if the intensity of an acoustic wave exceeds a certain limit.
However, the time of conversion should be decreased as much as possible to minimise losses of light radiation. Since a certain number of periods of an acoustic wave are required for a given conversion, the time of conversion decreases as the period of an acoustic wave decreases and its frequency increases. The same conclusion can be obtained from expression (3). Actually, as F increases, xcex9 decreases, the total length of the focon decreases and the total time of conversion decreases. Furthermore, total dissipate losses decrease because they are proportional to the time of light propagation or the time of conversion. However, as follows from (2) and (3), a decrease in xcex9, requires an increase in "sgr" and m.
The optimal frequency F of an acoustic wave depends on many factors. In particular, it depends on the definition of the word xe2x80x9coptimalxe2x80x9d in a given situation. Considerations for choosing the optimal F are discussed in a paper by V. P. Torchigin and A. V. Torchigin. Enlarged shift of light frequency in a modified Bragg cell. Pure and Applied Optics, vol. 7, pp.763-782 (1998). Analysis shows that, usually, F≅50 MHz. As is discussed by Torchigin in the Russian patent, acoustic waves form open dielectric resonators not only in a glass focon with a circular cross section but also in structures with other cross sections, such as, for, example, pyramids with square or hexagonal bases.
Thus, in a glass focon, travelling acoustic waves form open dielectric resonators that move at the velocity of the acoustic wave. As an open dielectric resonator moves towards the narrow part of a focon its cross-section decreases and the light radiation contained within it is compressed. As a result, its energy and frequency increase.
Various explanations of these facts are as follows:
1. Light radiation exerts pressure on the conical sides of the resonator. As the cross section of the resonator decreases, mechanical work is required to overcome the pressure. This mechanical work is exactly equal to the increase in light radiation energy in the resonator.
2. As the conical surface of the resonator decreases, a Doppler effect develops with each reflection of a light wave from the moving decreasing surface. This results in an increase in the frequency of the reflected wave. As a consequence of repeated reflections the frequency of the light radiation can increase significantly.
3. The concurrent increase in energy and frequency of the light radiation stored in the resonator is easily explained. The total number of photons in the resonator is unchanged during the process of compression if dissipative losses are negligible. Since the total energy in the resonator increases, the energy of each photon also increases. Because E=hv, where E is the energy of a photon, and v is its frequency, an increase of E proportionally increases v.
4. In any resonator, the resonator mode of electromagnetic oscillations is preserved on slow (adiabatic) compression of the resonator. That is, the number of spatial variations of the electromagnetic field along co-ordinates of the resonator is unchanged. In this situation, if the diameter of the cross-section of the resonator decreases by K times then the wavelength of the xe2x80x9cwhispering galleryxe2x80x9d type travelling wave also decreases by K times. And, it follows that the frequency of the light wave increases by K times. In accordance with Manley-Rowe relations, the energy of light radiation stored in the resonator increases proportionally to the increase of its frequency, i.e. by K times in the given situation. The index of frequency conversion is determined from the following simple relation.
D1/D2=xcex1/xcex2, xe2x80x83xe2x80x83(4)
where D1 is the diameter of the cross-section at the point of input of light radiation, D2 is the diameter of the cross-section at the point of output of that light radiation, xcex1 is the wavelength of the input light, xcex2 is the wavelength of the output or converted light. For, example, if D1/D2=2 then the frequency of the converted light increases by an octave, i.e. the second harmonic is obtained. In this case, where D1/D2=2, it follows that the frequency of the converted or output light is equal to two times the frequency of the input light.
5. The maximum energy density of light radiation in a focon does not coincide with the maximum acoustic pressure or the maximum refractive index. The maximum energy density is located to the left of the maximum acoustic pressure when the narrow part of the focon is located to the right. The maximum energy density is located in the region of the focon where the acoustic pressure is decreasing with time as the wave propagates towards the narrow end of the focon. On the other hand, it is known that, due to an electrostriction effect, a light wave generates a pressure in the medium where it propagates. Thus, the pressure of the light wave is increasing as the pressure of the acoustic wave is decreasing, i.e. the action of the acoustic wave is directed against the action of the light radiation. As a result, an interchange of the energies between the acoustic and light waves takes place. Acoustic wave energy is transferred into light wave energy.
6. When a WGM light beam propagates towards the tapered part of a focon its axial component decreases on each reflection from any side plane of the focon. As a result, the angle between the light beam and the axis of the focon increases. When the angle becomes equal to 90xc2x0 the shift of the beam towards the narrow end of the focon terminates and the beam achieves a turning point. After the turning point, the beam propagates towards the wide part of the focon with a progressively increasing angle.
On the other hand, when an acoustic wave is propageted along the axis towards the narrow (tapered) end of the focon, a light wave reflected at a turning point propagates in the opposite direction, towards the base of the focon. The angle between the direction of the light beam and the axis decreases as the light beam travels towards the base. There is a point where the corresponding angle is such that the Bragg condition is valid. In this case, a light wave reflects from a so-called distributed Bragg reflector (DBR) formed by an acoustic wave and propagates in the direction of the taper (narrow part) of the focon. It is known that the frequency of a diffracted light wave, reflected from a moving DBR, increases by F, the frequency of the acoustic wave. A light wave that reaches the turning point reflects from it and propagates again to meet the DBR formed by the acoustic wave. Upon meeting the fragment of the acoustic wave that is its corresponding DBR, the light wave reflects from it again. As this takes place, the frequency of the light wave increases by F again and becomes equal to f+2F.
Thus, the frequency of the light wave increases by F on each reflection from the DBR but is unchanged on each reflection at the turning point. As a result of repeated reflections, the frequency of the light wave can increase significantly. In this case, the cascading multistage frequency conversion in the same device follows the scheme fxe2x86x92f+Fxe2x86x92f+2Fxe2x86x92 . . . xe2x86x92f+NF. Notably, there always exists at least one acoustic wave fragment that functions as a DBR for light waves having the appropriate frequency. At the same time, there are turning points for light waves of various frequencies. Therefore, the conditions necessary for repeated increasing of the light frequency on the repeated reflections from the DBR are satisfied for light waves with various frequencies. The light frequency in an ideal system without dissipative and radiative losses increase when the accepted assumptions are valid.
Thus, in this case (i.e. a focon) we have an open dielectric resonator in that the propagation of light radiation is confined along the axis by the turning point and the DBR, where the turning point is farther from the base of the focon than the DBR is from the base of the focon. As the frequency of the light wave increases, both the turning point and the DBR are shifted toward the tapered end of the focon. That is, the resonator moves towards the tapered end of the focon. Analysis shows that the velocity of the resonator movement equals the velocity of the acoustic wave.
From the foregoing considerations, one can conclude that the energy and frequency of the light radiation stored in the resonator change proportionally to one other. When loses in a resonator are negligible during compression of a light wave, the increase in energy and frequency is determined by the simple relation (4).
Thus, the peculiarities of the presented method of light conversion are the following.
1. Since the conversion of light radiation frequency occurs gradually as light is propagated along the axis of the focon, it is possible to obtain output radiation with various frequencies. In order to obtain light radiation outputs of more than one frequency, it is sufficient to place several coupling prisms along the side surface of the focon.
2. Several light signals with various frequencies can be amplified and converted simultaneously.
3. The wavelength range for input and output light radiation is broad and is limited only by light attenuation of the medium used in the focon.
4. The availability of types of glass with small losses for light and acoustic waves and with high resistance to intensive light and acoustic waves permit the creation of light amplifiers-converters and generators having small size, high efficiency, and large output power.
5. Components and materials used in the devices disclosed in this application are employed in modern acoustooptics and are commercially available.
6. The parameters of some types of available glass permit conversion and amplification in the wavelength range of one octave.
7. Amplification and conversion can be performed in a linear medium, for example, in the glass used for fabrication of fibers.
8. There are no critical parameters or sizes. That is, the device can consist of a tapered fiber having a small diameter or a glass body having a large diameter. Conversion can take place not only in any glass focon but also, as disclosed in this application, in any glass body having sufficiently good homogeneity wherein the condition "sgr" less than m is satisfied, Homogeneity is sufficiently good when deflections from the medium value of the refractive index are less than the index of modulation, that is, less than 10xe2x88x924.
The device disclosed in Patent 2085984RU has certain drawbacks that are overcome by the invention presented in this disclosure. For example, relative to patent 2085984RU, the invention of the present disclosure increases the index of light frequency conversion and the efficiency of conversion. Furthermore, in the device disclosed in patent 2085984RU, the side surface of the device that forms the air-glass boundary from which the WGM is totally reflected degrades with time and the parameters of the device worsen.
Parametric Optical Devices
A parametric optical apparatus uses the mechanical energy of elastic oscillations to convert light over a wide range of wavelengths and amplitudes and to generate coherent light. The present invention relates to optical and acoustooptical devices and more particularly to parametric amplifiers, converters, and generators of light radiation. In certain applications, the apparatus is an alternative to the use of lasers and converters.
Each of the optical parametric devices presented in this invention take advantage of at least one of the peculiarities of wavelength conversion, amplification and generation of light waves that occur during the interaction between light and acoustic waves in a lightguide with a cross-section that varies along its axis. The wavelength conversion, amplification and generation of light waves disclosed in the present invention are not typical of the interaction between light and acoustic waves and were not expected prior to the experiments of the inventor.
An object of the present invention is to provide a parametric optical apparatus with acoustic pump wherein relatively cheap energy of elastic oscillations is transformed into energy of coherent light in a broad range of light wavelengths and powers.
Another object of the present invention is to provide a parametric optical apparatus with acoustic pump that is useful in various specific fields of applications.
A specific object of the present invention is to improve the efficiency and operating characteristics of the prior-art amplifier-converters of light radiation.
A further object of the present invention is to provide a parametric generator of coherent light based on said parametric optical apparatus wherein coherent light is obtained from the energy of an acoustic pump without any additional sources of light. When dissipate losses from the glass are small, a broad range of wavelengths and power of output light radiation can be obtained.
The present invention provides a first parametric optical apparatus with acoustic pump wherein a glass body of rotation is used in which the refraction index in the cross section of the body decreases as the distance from the axis increases. This decreases losses of light radiation due to the scattering of light by various imperfections on the conical side surface of the body of rotation.
The present invention provides a second parametric optical apparatus with acoustic pump wherein the length of said body of rotation is extended to function as a matching load for the acoustic wave. This simplifies fabrication of the body and improves the quality of matching.
The present invention further provides a third parametric optical apparatus with acoustic pump wherein the acoustic transducer is fabricated from piezoceramic. An acoustic wave is excited by single acoustic pulses with a low frequency of repetition. This decreases the total power of the source for exciting the acoustic wave.
The present invention still further provides a fourth parametric optical apparatus with acoustic pump wherein a series of acoustic pulses is used instead of a single acoustic pulse to increase the efficiency of conversion by means of decreasing the length of the focon.
The present invention provides a fifth parametric optical apparatus with acoustic pump wherein a series of pulses with gradually increasing frequency is used to excite an acoustic wave.
The present invention further provides a sixth parametric optical apparatus with acoustic pump wherein the frequency of output light radiation depends on the power of the acoustic wave, that is, the frequency of the output light radiation can be changed by changing the electrical power exciting the acoustic transducer. This provides a means for a rapid (electronic speed) change of frequency of the output light radiation.
The present invention still further provides a seventh parametric optical apparatus with acoustic pump wherein the amplification and frequency conversion of non-coherent light radiation with various wavelengths can be performed simultaneously. This simplifies the input of light radiation.
The present invention still further provides an eighth parametric optical apparatus with acoustic pump wherein a surface acoustic wave is excited in said body. This decreases the acoustic power required.
The present invention still further provides a ninth parametric optical apparatus with acoustic pump wherein specific coupling devices in the form of focons are used. This permits the index of coupling to be adjusted without any mechanical shifts of the coupling devices.
The present invention still further provides a tenth parametric optical apparatus with acoustic pump without an acoustic transducer for some applications. This significantly simplifies the apparatus.
The present invention still further provides an eleventh parametric optical apparatus with acoustic pump wherein the wavelength of the output radiation is changed by means of changing the power of the input radiation. This enables one to change (or control) the output light wavelength with a change of input light power.
The present invention still further provides a twelfth parametric optical apparatus with acoustic pump wherein a glass cone with a barrel-shaped segment is used. This is useful in some applications as the input of light radiation is simplified.
The present invention still further provides a thirteenth parametric optical apparatus with acoustic pump wherein a conical glass capillary tube is used as the body in which the interaction between acoustic and light waves takes place and in which an azimuth independent acoustic Lamb wave with one variation of pressure along the radius is excited (instead of a plane longitudinal acoustic wave) in the body. This increases the phase velocity of the acoustic wave and thereby decreases the time of conversion and, therefore, increases the efficiency of conversion.
The present invention still further provides a fourteenth parametric optical apparatus with acoustic pump wherein a conical capillary tube with variable thickness of its walls is used. This increases the efficiency of conversion.
The present invention still further provides a fifteenth parametric optical apparatus with acoustic pump wherein a glass cone with homogeneous refraction index is used instead of an inhomogeneous glass cone. In this case, the losses of conversion increase but problems of input/output light radiation in/out of the cone are simplified.
The present invention still further provides a sixteenth parametric optical apparatus with acoustic pump wherein a lightguide with a regular polygon or rectangular cross section is used instead of a circular cross-section. This further simplifies the problems related to the input and output of light radiation into and out of the lightguide.
The present invention still further provides a seventeenth parametric optical apparatus that uses a coherent light generator based on the energy of the acoustic wave in conjunction with other selected apparatuses of this invention. A feedback loop comprising a light frequency divider on the base of a nonlinear crystal is introduced for this purpose. The divider divides the light frequency by two.
The present invention provides an eighteenth parametric optical apparatus with acoustic pump wherein the first to seventeenth apparatuses disclosed provide a generator of coherent light with various wavelengths that can be used for forming WDM signals. Several output coupling devices are located along the lightguide for this purpose.
The present invention provides a nineteenth parametric optical apparatus with acoustic pump wherein a glass coating is used in which the refraction index of the glass coating is less than the refrection index of the glass body. This decreases losses of light radiation due to the scattering of light by various imperfections on the conical side surface of the body.
Acouto-Optical Applications
The present invention relates to acousto-optic devices, and particularly to acousto-optic devices with a response time of about several microseconds, such as switches, modulators of light radiation, acousto-optic tuneable filters, deflectors and light beam scanners. These devices take advantage of parametric optical apparatuses that use the mechanical energy of elastic oscillations to convert light over a wide range of wavelengths and amplitudes and to generate coherent light.
A primary objective of the present invention is to provide an apparatus based on an acousto-optic cell with variable cross-section that functions as a gate of light radiation controlled by an acoustic wave. Another object of the present invention is to provide an acousto-optic tuneable filter that can be useful in various specific fields of applications. Both devices have a similar design and are based on the same physical effects. A further object of the present invention is to provide an apparatus based on an acousto-optic cell with variable cross-section for the deflection of a light beam and for scanning. Another object of the present invention is to provide an apparatus based on an acousto optic cell with variable cross-section for coupling controlled by the acoustic wave between input and output ports. In particular when the ports are fibers.
The present invention provides a first acousto-optic apparatus wherein a gate of light radiation controlled by an acoustic wave is realised. The acoustic wave reflects the light wave propagated in the waveguide with variable cross-section. The reflected light wave is used as an output signal of the gate. The gate operates with light waves of various polarisations and wavelengths.
The present invention provides a second acousto-optic apparatus wherein a light radiation switch is controlled by an acoustic wave. In the absence of an acoustic wave, an input light wave propagates from an input port to an output port. When an acoustic wave is present, the acoustic wave reflects the light wave that propagates from an input port to an output port. The switch can operate with light waves of various polarisations and wavelengths.
The present invention provides a third acousto-optic apparatus wherein the wide face or base of the AO cell is partially free of an acoustic transducer and the cell is used as a reflector. The presence of an acoustic wave in the AO cell changes the properties of the reflector, in particular, the angle of reflection. This property is used for deflection, scanning, and switching light beams.
The present invention provides a fourth acousto-optic apparatus wherein the AO cell has a segment with minimal diameter. An output coupling device is placed after the segment. In an AO cell, an acoustic wave forms a distributed Bragg reflector that reflects light waves with wavelengths less than a certain boundary of frequencies. A change of the frequency of the acoustic wave changes the boundary. This effect is used in acousto optic tuneable filters (AOTF) to control the light wavelengths with the frequency of the acoustic wave.
The present invention provides a fifth acousto-optic apparatus that features the connecting of an AO cell with a waveguide. The small cross-section of an AO cell permits the direct coupling of a fiber device with the AO cell. Such coupling devices use common evanescent fields for coupling. These fiber, waveguide-like, coupling devices simplify coupling relative to the use of lens or prism coupling devices.
The present invention provides a sixth acousto-optic apparatus wherein a surface azimuth-independent acoustic wave, instead of a volume acoustic wave, is excited on the side surface of the AO cell. Use of a surface wave decreases the power of the acoustic wave required.
The present invention provides a seventh acousto-optic apparatus wherein a matching load for the acoustic wave is excluded to simplify fabrication of the AO cell. The acoustic wave attenuates in the additional segment of the lengthened AO cell. The AO cell is fabricated with the same technology that is used for fabrication of glass fibers and, therefore, the additional region of the AO cell can be fabricated without noticeable additional effort.
The present invention provides an eighth acousto-optic apparatus wherein a conical capillary tube is used as the waveguide. Since the area of the cross-section of a capillary tube is less than the area of an analogous cross-section of a solid or continuous cone, the use of a capillary tube decreases the power of the acoustic wave required and increases the efficiency of the interaction between the acoustic and light waves due to an increase of their overlapping.
The present invention provides a ninth acousto-optic apparatus wherein an azimuth-independent symmetrical acoustic Lamb wave with one variation of pressure along the radius of a conical capillary tube is excited instead of a plane longitudinal acoustic wave. This increases the index of modulation of the refractive index without increasing the power of the acoustic wave and consequently decreases the power of the acoustic wave required.
The present invention provides a tenth acousto-optic apparatus wherein the refraction index of the core of the waveguide is greater than the refractive index of the coating of the waveguide. This decreases losses of the light wave due to defects and imperfections on the side surface of the waveguide.
The present invention provides an eleventh acousto-optic apparatus wherein the cross section of the waveguide has the form of a regular polygon or rectangle. This enables the use of an AO cell with a traditional cross section in applications where such form is preferable.